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Black Spot on Roses

There are three common fungus problems that affect roses: Black Spot, Powdery Mildew and Cercospora Leaf Spot. Powdery Mildew is addressed in a separate info-sheet (read about Powdery Mildew here). Cercospora Leaf Spot (Images 1 and 2), look like most leaf spot fungus with a dark margin and a tan or grey center. This leaf spot starts as a purplish spot then the center turns color as the leaf begins to die. Control is the same as Black Spot, both in timing and chemical.

Black Spot (Images 3 and 4) is often an irregular spot with feathery borders. Yellowing may appear around the spots, (Image 4, but is starting in Image 3 as well). Leaves are the most susceptible, but spots can be seen on other parts of the plant including the stem (Image 5).

Infection usually starts on the lower leaves and works its way up the plant (Image 6). As the yellow proceeds across the leaves, they will eventually drop, leaving the bottom of the plant bare with only a few leaves at the top (Image 7). This defoliation decreases the plant’s energy reserves, reduces flowering and weakens the plant making it more susceptible to other stress factors such as drought and winter injury. Black Spot will be most severe when humidity is high, over 85%, when warm temperatures are over 75° and with six or more hours of leaf wetness.

Control

The first line of defense is to try some cultural controls: avoid planting susceptible varieties; keep irrigation water off the foliage; plant in full sun with good air movement. Also, try sanitation by removing diseased leaves that have fallen and prune out infected rose canes. Spray if needed.

ferti•lome® Liquid Systemic Fungicide is a systemic fungicide to be applied at 10 day intervals. Another systemic option is ferti•lome® 2-N-1 Systemic which has a fungicide and an insecticide and can be used as a drench– no spraying!